Nonvolatile semiconductor device and method for testing the same

ABSTRACT

A nonvolatile semiconductor device and a method for testing the same are provided. The nonvolatile semiconductor device includes a current generating unit configured to generate a set write current depending on a step pulse that is generated based on a reference current and output the set write current to a memory cell, and a current measuring unit configured to measure a step duration of the step pulse and output a measured result outside of a chip during an activation period of a test enable signal.

CROSS-REFERENCE TO RELATED APPLICATION

The priority of Korean patent application No. 10-2012-0047419 filed onMay 4, 2012 the disclosure of which is hereby incorporated in itsentirety by reference, is claimed.

BACKGROUND OF THE INVENTION

The present invention relates to a nonvolatile semiconductor device anda method for testing the same, and more specifically, to a technologyfor controlling a write operation of a nonvolatile memory deviceconfigured to sense data based on resistance change.

Generally, memory devices may be classified as volatile memory devicesor nonvolatile memory devices. A nonvolatile memory is device includes anonvolatile memory cell capable of preserving stored data even when apower source is off. A nonvolatile memory device may be implemented, forexample, as a flash random access memory (RAM) device, a phase changeRAM (PCRAM) device, or the like.

PCRAM devices include memory cells that are implemented using a phasechange material, for example, germanium antimony tellurium (GST), andare configured to store data in the memory cells by applying heat to theGST so that the GST changes into a crystalline phase or an amorphousphase.

A nonvolatile memory device, such as a magnetic memory device, a phasechange memory (PCM) device, or the like, has a data processing speedsimilar to that of a volatile RAM device. A nonvolatile memory devicealso preserves data even when a power source is off.

FIGS. 1 a and 1 b illustrates a conventional phase change resistancedevice 4.

Referring to FIGS. 1 a and 1 b, a conventional phase change resistancedevice 4 includes an upper electrode 1, a lower electrode 3, and a phasechange material 2 interposed between the upper electrode 1 and the lowerelectrode 3. When a voltage is applied to the upper electrode 1 and thelower electrode 3, a current flows into the phase change material 2,thus inducing a high temperature in the phase change material 2. As aresult, the electrical conductive state of the phase change material 2changes depending on resistance variation due to the high temperature.

FIGS. 2 a and 2 b illustrates a phase change principle of theconventional phase change resistance device 4.

Referring to FIG. 2 a, if a low current smaller than a critical valueflows into the phase change resistance device 4, the phase changematerial 2 is crystallized. When the phase change material 2 changesinto a crystalline phase, it becomes a low resistance material. As aresult, a current can flow between the upper electrode 1 and the lowerelectrode 3.

On the other hand, referring to FIG. 2 b, if a high current greater thanthe critical value flows into the phase change resistance device 4, thephase change material 2 has a temperature higher than a quenching point.When the phase change material 2 changes into an amorphous phase, i.e.,a non-crystalline phase, it becomes a high resistance material. As aresult, a current cannot easily flow between the upper electrode 1 andthe lower electrode 3.

The phase change resistance device 4 can store data corresponding to tworesistance phases. That is, in one case, if a low resistance phase inthe phase change resistance device corresponds to a data “1,” and thehigh resistance phase corresponds to a data “0,” then the phase changeresistance device 4 may store two logic states of data.

This data can be stored in the phase change resistance device 4 asnonvolatile data because the status of the phase change material 2 doesnot change even when a power source is off.

FIG. 3 illustrates a write operation of a conventional phase changeresistance cell.

Referring to FIG. 3, heat is generated if a current flows between theupper electrode 1 and the lower electrode 3 of the phase changeresistance device 4 for a given time.

If a low current, smaller than the critical value, flows for the giventime, the phase change material 2 changes into a crystalline phase. As aresult, the phase change resistance device 4 becomes a low resistanceelement having a set phase.

On the other hand, if a high current, greater than the critical value,flows for a given time, the phase change material 2 changes into anamorphous phase. As a result, the phase change resistance device 4becomes a high resistance element having a reset phase.

Accordingly, a low voltage is applied to the phase change resistancedevice 4 for a long period of time in order to write the set phase inthe write operation.

On the other hand, a high voltage is applied to the phase changeresistance device 4 for a short period of time in order to write thereset phase in the write operation.

To change the phase change resistance cell into the set phase, it isimportant to control a quenching slope of a set write current, which isrequired for crystalizing the phase change resistance cell, by graduallyreducing an amount of the set write current. This way of graduallyreducing the set write current is called “quenching.”

However, for example, if a reference current, which is used to generatethe set write current and is received from the outside or generatedinside the chip, changes by some factors or if a clock having a wrongvalue is generated by mismatched circuits, the set write current may begenerated to have an undesired quenching slope.

Moreover, without accurately checking the quenching slope, it isimpossible to precisely control the phase change resistance cell inprogram and verify (PNV) operations and a multi-level cell MLC wheremulti-leveled resistance distribution of a phase change material, e.g.,germanium antimony tellurium (GST), is formed.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to providing atechnology for measuring a quenching slope and a step duration at theoutside of a chip in case a set write current is supplied to a phasechange resistance cell in a write operation so as to control the writecurrent more precisely.

According to an embodiment of the present invention, a nonvolatilesemiconductor device comprises: a current generating unit configured togenerate a set write current depending on a step pulse that is generatedbased on a reference current and output the set write current to amemory cell; and a current measuring unit configured to measure a stepduration of the step pulse and output the step duration outside of achip during an activation period of a test enable signal.

The current measuring unit comprises a test control unit configured tomeasure the step duration to detect a change in a quenching slope of theset write current as a pulse.

The test control unit is configured to measure the step duration andoutput a first step enable pulse representing a first step pulseduration of the step pulse and a full step enable pulse representing apulse duration of a whole step of the step pulse.

The current measuring unit further comprises an output unit configuredto output the first step enable pulse and the full step enable pulseoutside of the chip.

The output unit comprises: a first output pad configured to output thefirst step enable pulse in response to the test enable signal; and asecond output pad configured to output the full step enable pulse inresponse to the test enable signal.

The test control unit comprises: a plurality of quenching time measuringunits configured to measure the step duration of the step pulse; and apulse generating unit configured to generate the first step enable pulseand the full step enable pulse based on output signals of the pluralityof the quenching time measuring units and output the first step enablepulse and the full step enable pulse during the activation period of thetest enable signal.

Each of the plurality of quenching time measuring units comprises: apulse driving unit configured to drive the step pulse in response to anenable signal; and a latch unit configured to latch an output signal ofthe pulse driving unit.

The current measuring unit further comprises a delay unit configured todelay an initial enable signal to control an enable period of thecurrent measuring unit.

The current generating unit comprises: an oscillator configured togenerate a clock for determining a quenching duration of the set writecurrent based on the reference current; a clock counter configured tocount the clock and output a count signal; a step pulse generating unitconfigured to generate the step pulse in response to the count signal;and a write current generating unit configured to generate the set writecurrent depending on the step pulse.

The set write current maintains a given current value for a given timeat an initial state and then changes to a current value that isdecreased in a step form in response to the step pulse.

A first step period of the set write current has a given duration.

The current measuring unit measures the step duration using a first steppulse corresponding to a period from an initial state until a currentvalue of the set write current starts to decrease in a step form, asecond step pulse corresponding to a period ending when the set writecurrent decrease by one step, and a third step pulse corresponding to aperiod from the initial state until the current value of the set writecurrent decrease by a whole step.

According to another embodiment of the present invention, a method fortesting a nonvolatile semiconductor device comprises: generating a setwrite current depending on a step pulse that is generated based on areference current and outputting the set write current to a memory cell;and measuring a step duration of the step pulse and outputting the stepduration outside of a chip during an activation period of a test enablesignal.

The method further comprises measuring a quenching slope change of theset write current at the outside of the chip based on the step duration.

Detecting a high level or a low level of the step duration by applying atime duration to the step duration outputted through an output pad andchecking a toggling point to measure the quenching slope change.

The method further comprises converting the quenching slope change ofthe set write current into a pulse.

The method further comprises outputting a first step enable pulserepresenting a first step pulse duration of the step pulse and a fullstep enable pulse representing a pulse duration of whole steps of thestep pulse.

The method further comprises: generating a clock for determining aquenching duration of the set write current based on the referencecurrent; counting the clock to output a count signal; generating thestep pulse in response to the count signal; and generating the set writecurrent depending on the step pulse.

The set write current maintains a given current value for a given timeat an initial state and then changes to a current value that isdecreased in a step form in response to the step pulse.

A first step period of the set write current has a given duration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 a and 1 b illustrate a conventional phase change resistancedevice.

FIGS. 2 a and 2 b illustrate a phase change principle of theconventional phase change resistance device.

FIG. 3 illustrates a write operation of a conventional phase changeresistance cell.

FIG. 4 illustrates a nonvolatile semiconductor device according to anembodiment of the present invention.

FIG. 5 illustrates a waveform diagram of a set write current accordingto an embodiment of the present invention.

FIG. 6 illustrates a waveform diagram of the set write current having acontrolled quenching time according to an embodiment of the presentinvention.

FIG. 7 illustrates a circuit diagram of a test control unit in FIG. 4.

FIG. 8 illustrates a waveform diagram of input pulses of the testcontrol unit in FIG. 4.

FIG. 9 illustrates a timing diagram of the test control unit in FIG. 7.

FIG. 10 illustrates input/output pad signals in a nonvolatilesemiconductor device according to an embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

Exemplary embodiments of the present invention will be described indetail with reference to the attached drawings.

FIG. 4 illustrates a nonvolatile semiconductor device according to anembodiment of the present invention.

The nonvolatile semiconductor device includes a current generating unitand a current measuring unit.

The current generating unit includes an oscillator 100, a clock counter200, a step pulse generating unit 300, and a write current generatingunit 400. The current measuring unit includes a delay unit 500, a testcontrol unit 600, and an output unit 700.

The oscillator 100 oscillates a reference current REF during anactivation period of an initial enable signal ENIN so as to generate aclock CLK having a quenching duration.

The oscillator 100 generates the clock CLK having a given cycle in orderto control a set write current SWC required for crystalizing a phasechange resistance cell.

The clock counter 200 counts the clock CLK received from the oscillator100 and encodes a count result to output count signals CNT<0:2>.

The step pulse generating unit 300 decodes the count signals CNT<0:2>and outputs step pulses STEP<0:7> to the write current generating unit400. The step pulses STEP<0,1,7> of the step pulses STEP<0:7> areprovided to the test control unit 600.

The write current generating unit 400 controls driving transistorstherein in response to the step pulses STEP<0:7> so as to supply the setwrite current SWC having a desired slope to the phase change resistancecell.

The set write current SWC has a current value that decreases in a stepform depending on the step pulses STEP<0:7>. A slope of the set writecurrent SWC is determined by the reference current REF inputted to theoscillator 100.

The delay unit 500 delays the initial enable signal ENIN to output anenable signal EN to the test control unit 600.

The test control unit 600 outputs a first step enable pulse STEP_1 and afull step enable pulse STEP_F in response to the step pulsesSTEP<0,1,7>, the enable signal EN, and a test enable signal TEN.

That is, the test control unit 600 measures and drives the duration ofthe step pulses STEP<0,1,7> when the test enable signal TEN isactivated.

The test control unit 600 converts a quenching slope change of the setwrite current SWT into a pulse and outputs the first step enable pulseSTEP_1 and the full step enable pulse STEP_F.

The output unit 700 includes output pads DQ<0:1>. The output unit 700outputs the first step enable pulse STEP_1 and the full step enablepulse STEP_F, which correspond to (A) and (B) of FIG. 4, respectively,during an activation period of the test enable signal TEN.

The first step enable pulse STEP_1 and the full step enable pulse STEP_Fmay be asynchronously transmitted to the output pads DQ<0:1>.

The first step enable pulse STEP_1 is outputted through the output padDQ0. The first step enable pulse STEP_1 as a pulse having a shortquenching time, as shown in (A) of FIG. 4, represents a pulse width ofone step of the step pulse.

The full step enable pulse STEP_F is outputted through the output padDQ1. The full step enable pulse STEP_F as a pulse having a longquenching time, as shown in (B) of FIG. 4, represents a total pulsewidth of steps of the step pulse.

That is, a result checked outside of a chip represents a time durationof a pulse where the set write current SWC is quenched.

In an embodiment of the present invention, a quenching slope of the setwrite current SWC is checked in a test operation. If a test code isinputted, the step enable pulses STEP_1 and STEP_F outputted through theoutput pads DQ<0:1> may be checked outside of the chip.

FIG. 5 illustrates a waveform diagram of the set write current accordingto an embodiment of the present invention.

The set write current SWC outputted from the write current generatingunit 400 has a current value that decreases in a step form as shown inFIG. 5. A given current is supplied in an initial phase for a giventime, and then driving transistors in the write current generating unit400 are sequentially turned off to control the current in the step form.

Since the set write current SWC is generated using the clock CLK fromthe oscillator 100, the duration of each step of the set write currentSWC is constant.

The resistance distribution of the phase change resistance cell isdifferentiated depending on a slope of quenching slew generated by theset write current SWC that has a current value decreasing in the stepform .

In the waveform diagram of FIG. 5, the duration of one step where theset write current SWC is quenched is defined as “1STEP”, and the wholequenching time is defined as “QT”.

FIG. 6 illustrates a waveform diagram of a set write current having acontrolled quenching time according to an embodiment of the presentinvention.

Referring to FIG. 6, the quenching time of the set write current SWC maybe controlled to, e.g., QT1, QT2, or QT3. As the quenching time becomeslonger as shown by QT1, QT2, and QT3, “1STEP”, which is the duration ofone step where the set write current SWC is quenched and which is shownin FIG. 5, also becomes longer.

The quenching duration of the set write current SWC may bedifferentiated by the reference current REF inputted to the oscillator100.

In the prior art, the reference current REF inputted to the oscillator100 is controlled to regulate the quenching time for crystalizing thephase change resistance cell. However, it is impossible to check whethera desired quenching slope is generated or not in an actual wafer.

On the other hand, in accordance with an embodiment of the presentinvention, it is possible to measure and control a write current thatchanges a phase of a phase change material in a write operation.

The state of the phase change material, such as a chalcogenide compound(e.g., Ge—Sb—Te (GST)), is changed by heat generated by a currentsupplied to the phase change material.

If a phase change resistance cell using the GST is changed into a setphase, a set write current is gradually reduced so that the GST iscrystallized.

In order to change the phase change resistance cell into the set phase,it is important to control the quenching slope of the set write currentby gradually reducing an amount of the set write current.

In accordance with an embodiment of the present invention, the quenchingslope and the step duration can be measured outside of the chip to checkand readjust the relationship between the set write current and thephase change of the GST. As a result, the resistance distribution of GSTcells may be accurately adjusted.

FIG. 7 illustrates a circuit diagram of the test control unit 600 inFIG. 4.

The test control unit 600 includes a plurality of quenching timemeasuring units 610-630 and a pulse generating unit 640.

The quenching time measuring unit 610 includes a pulse driving unit 611and a latch unit 612. The pulse driving unit 611 includes a plurality ofPMOS transistors P1 and P2 and a plurality of NMOS transistors N1˜N3.

The PMOS transistors P1 and P2 and the NMOS transistors N1 and N2 arecoupled in series between a power voltage terminal and a ground voltageterminal. The PMOS transistor P1 has a gate to receive an enable signalEN. The enable signal ENb is an inversion signal of the enable signalEN. The PMOS transistor P2 and the NMOS transistor N1 receive the steppulse STEP0 through their gates, which are coupled to each other.

The NMOS transistor N2 has a gate to receive the enable signal ENb.

The NMOS transistor N3 coupled between a node (C) and the ground voltageterminal has a gate to receive the enable signal ENb. The node (C) iscoupled to drains of the PMOS transistor P2 and the NMOS transistor N1.

The latch unit 612 includes inverters IV2 and IV3 coupled to each otherwith a latch type between the node (C) and a node NODE1.

The quenching time measuring unit 620 includes a pulse driving unit 621and a latch unit 622. The pulse driving unit 621 includes a plurality ofPMOS transistors P4 and P5 and a plurality of NMOS transistors N4˜N6.

The PMOS transistors P4 and P5 and the NMOS transistors N4 and N6 arecoupled in series between the power voltage terminal and the groundvoltage terminal. The PMOS transistor P4 has a gate to receive theenable signal EN. The PMOS transistor P5 and the NMOS transistor N4receive the step pulse STEP1 through their gates, which are coupled toeach other.

The NMOS transistor N5 has a gate to receive the enable signal ENb. TheNMOS transistor N6 coupled between a node (D) and the ground voltageterminal has a gate to receive the enable signal ENb. The node (D) iscoupled to drains of the PMOS transistor P5 and the NMOS transistor N4.

The latch unit 622 includes IV5 and IV6 coupled to each other in a latchform between the node (D) and a node NODE3.

The quenching time measuring unit 630 includes a pulse driving unit 631and a latch unit 632. The pulse driving unit 631 includes a plurality ofPMOS transistors P7 and P8 and a plurality of NMOS transistors N7˜N9.

The PMOS transistors P7 and P8 and the NMOS transistors N7 and N8 arecoupled in series between the power voltage terminal and the groundvoltage terminal. The PMOS transistor P7 has a gate to receive theenable signal EN. The PMOS transistor P8 and the NMOS transistor N7receive the step pulse STEP7 through their gates, which are coupled toeach other.

The NMOS transistor N8 has a gate to receive the enable signal ENb. TheNMOS transistor N9 coupled between a node (E) and the ground voltageterminal has a gate to receive the enable signal ENb. The node (E) iscoupled to drains of the PMOS transistor P8 and the NMOS transistor N7.

The latch unit 632 includes IV8 and IV9 coupled to each other with alatch type between the node (E) and a node NODE4.

The pulse generating unit 640 includes a plurality of invertersIV10˜IV12 and NAND gates ND1 and ND2. The inverter IV10 inverts anoutput signal of the latch unit 612 to provide an output signal througha node NODE2.

The NAND gate ND1 performs a NAND operation on the output signal of thenode NODE2, an output signal of the node NODE3, and the test enablesignal TEN. The inverter IV11 inverts an output signal of the NAND gateND1 to output the first step enable pulse STEP_1.

The NAND gate ND2 performs a NAND operation on the output signal of thenode NODE2, an output signal of the node NODE4, and the test enablesignal TEN. The inverter IV12 inverts an output signal of the NAND gateND2 to output the full step enable pulse STEP_F.

The step pulses STEP<0:7> applied to the write current generating unit400 are shown in FIG. 8.

Referring to FIG. 8, if the enable signal EN has a high level, the steppulses STEP<0:7> are synchronized with the enable signal EN andactivated to a high level.

Each subsequent activation period from STEP0 to STEP7 is graduallyenlarged so that they are sequentially inactivated to a low level inorder from STEP0 to STEP7.

If the enable signal EN has a low level, the final step pulse STEP7 issynchronized with the enable signal EN and inactivated to the low level.

The write current generating unit 400 controls the set write current SWCusing the step pulses STEP<0:7> that have different activation periodsas shown in FIG. 8.

The test control unit 600 measures the quenching duration of the setwrite current SWC based on the step pulses STEP<0,1,7> selected from thestep pulses STEP<0:7> and outputs measured results outside of the chip.

In an embodiment of the present invention, the three step pulses

STEP<0,1,7> of the step pulses STEP<0:7> are used to measure theduration of one step and the duration of total steps included in the setwrite current.

The quenching time is measured using the step pulse STEP0 representing aperiod where the set write current SWC does not decrease, the step pulseSTEP1 representing a period where the set write current SWC decreases byone step, and the step pulse STEP7 representing a period where the setwrite current SWC decreases from an initial step to a final step, i.e.,by the whole steps.

FIG. 9 illustrates a timing diagram of signals relating to the testcontrol unit 600 in FIG. 7.

In an initial phase, if the enable signal EN has a low level, i.e., theenable signal ENb has a high level, the NMOS transistors N3, N6, and N9in FIG. 7 are turned on. As a result, the nodes (C), (D), and (E) have alow level, and thus the nodes NODE2, NODE3, and NODE4 have a low level,a high level, and a high level, respectively.

After that, if the enable signal EN is enabled to a high level, the testcontrol unit 600 starts to measure the quenching time of the step pulsesSTEP<0,1,7>.

If the enable signal EN and the step pulse STEP0 move to a high level,the PMOS transistor P1 and the NMOS transistor N2 are turned on, and thePMOS transistor P2 is turned off while the NMOS transistor N1 is turnedon. At this time, the NMOS transistor N3 is turned off. As a result, thenode (C) maintains a low level, and the node NODE2 also maintains a lowlevel.

Subsequently, if the step pulse STEP0 moves to a low level, the PMOStransistor P2 is turned on while the NMOS transistor N1 is turned off,so that the node (C) moves to a high level, and the node NODE2 alsomoves to a high level. The node NODE2 maintains a high level for a latchtime of the latch unit 612.

If the enable signal EN moves to a low level again, i.e., the enablesignal ENb moves to a high level, the NMOS transistor N3 is turned on.Thus, the node (C) moves to a low level again, and the node NODE2 alsomoves to a low level.

In the same way, if the enable signal EN and the step pulse STEP1 moveto a high level, the PMOS transistor P4 and the NMOS transistor N5 areturned on, and the PMOS transistor P5 is turned off while the NMOStransistor N4 is turned on. At this time, the NMOS transistor N6 isturned off. Thus, the node (D) has a low level, and the node NODE3 has ahigh level.

Subsequently, if the step pulse STEP1 moves to a low level, the PMOStransistor P5 is turned on while the NMOS transistor N4 is turned off,so that the node (D) moves to a high level, and the node NODE3 moves toa low level. The node NODE3 maintains a low level for a latch time ofthe latch unit 622.

If the enable signal EN moves to a low level, i.e., the enable signalENb moves to a high level, the NMOS transistor N6 is turned on, so thatthe node (D) moves to a low level, and the node NODE3 moves to a highlevel.

Likewise, if the enable signal EN and the step pulse STEP7 move to ahigh level, the PMOS transistor P7 and the NMOS transistor N8 are turnedon, and the PMOS transistor P8 is turned off while the NMOS transistorN7 is turned on. At this time, the NMOS transistor N9 is turned off.Thus, the node (E) has a low level, and the node NODE4 has a high level.

Subsequently, if the step pulse STEP7 moves to a low level, the PMOStransistor P8 is turned on while the NMOS transistor N7 is turned off,so that the node (E) moves to a high level, and the node NODE4 moves toa low level.

If the enable signal EN moves to a low level, the NMOS transistor N9 isturned on. Then, the node (E) moves to a low level, and the node NODE4moves to a high level.

The pulse generating unit 640 performs an AND operation on the outputsignal of the node NODE2, the output signal of the node NODE3, and thetest enable signal TEN. The pulse generating unit 640 also performs anAND operation on the output signal of the node NODE2, the output signalof the node NODE4, and the test enable signal TEN.

As a result, in a period where all of the above output signals on thenodes NODE2 and NODE3 and the test enable signal TEN are enabled to ahigh level, the first step enable pulse STEP_1 having a high level isoutputted. In a period where all of the output signals on the nodesNODE2 and NODE4 and the test enable signal TEN are enabled to a highlevel, the full step enable pulse STEP_F having a high level isoutputted.

FIG. 10 illustrates input/output pad signals in a nonvolatilesemiconductor device according to an embodiment of the presentinvention. FIG. 10 shows a method of numerically measuring the quenchingduration of the set write current.

Pulse periods of the first step enable pulse STEP_1 and the full stepenable pulse STEP_F, outputted outside the chip through the output padsDQ0 and DQ1 of the output unit 700, are measured so that the quenchingduration of the set write current may be measured.

The quenching duration of the set write current is numerically measuredat the wafer level by detecting a high or low level of the measuredpulse by applying a time duration to the measured pulse and checking atoggling point.

For example, since the first step enable pulse STEP_1 is at a high levelfor a discrete short period, a high level period is represented as data“0” and a low level transition period is represented as data “1”.

In the same way, since the full step enable pulse STEP_F is also at ahigh level for a discrete short period, a high level period isrepresented as data “0” and a low level transition period is representedas data “1”.

By using this method, the quenching duration of the set write currentcan be checked numerically without using an oscilloscope. Moreover, itis possible to check the variation of the quenching duration of the setwrite current depending on a specific condition by changing a die in awafer.

Outside of the chip, the number of data “0” or “1” from the pulsesoutputted through the output pads DQ0 and DQ1 is determined, so that thequenching slope and the step duration of the set write current may bemeasured.

In accordance with an embodiment of the present invention, it ispossible to measure the change of the quenching slope and the stepduration of the write current depending on variation of the referencecurrent in the test operation and to check this at the package level soas to accurately control resistance distribution of the cells.

In addition, it is possible to verify the quenching slope and the stepduration of the write current of several dies under different conditions

As described above, in accordance with the present invention, thequenching slope and the step duration can be verified outside of a chipso as to check which change affects a desired quenching slope. As aresult, it is possible to form a phase change material having a desiredresistance distribution.

Although a number of illustrative embodiments consistent with thepresent invention have been described, it should be understood thatnumerous other modifications and embodiments can be devised by thoseskilled in the art that will fall within the spirit and scope of theprinciples of this disclosure. Particularly, numerous variations andmodifications are possible in the component parts and/or arrangementswhich are within the scope of the disclosure, the drawings, and theaccompanying claims. In addition to variations and modifications in thecomponent parts and/or arrangements, alternative uses will also beapparent to those skilled in the art.

What is claimed is:
 1. A nonvolatile semiconductor device, comprising: acurrent generating unit configured to generate a set write currentdepending on a step pulse that is generated based on a reference currentand output the set write current to a memory cell; and a currentmeasuring unit configured to measure a step duration of the step pulseand output the step duration outside of a chip during an activationperiod of a test enable signal.
 2. The nonvolatile semiconductor deviceaccording to claim 1, wherein the current measuring unit comprises atest control unit configured to measure the step duration to detect achange in a quenching slope of the set write current as a pulse.
 3. Thenonvolatile semiconductor device according to claim 2, wherein the testcontrol unit is configured to measure the step duration and output afirst step enable pulse representing a first step pulse duration of thestep pulse and a full step enable pulse representing a pulse duration ofa whole step of the step pulse.
 4. The nonvolatile semiconductor deviceaccording to claim 3, wherein the current measuring unit furthercomprises an output unit configured to output the first step enablepulse and the full step enable pulse outside of the chip.
 5. Thenonvolatile semiconductor device according to claim 4, wherein theoutput unit comprises: a first output pad configured to output the firststep enable pulse in response to the test enable signal; and a secondoutput pad configured to output the full step enable pulse in responseto the test enable signal.
 6. The nonvolatile semiconductor deviceaccording to claim 3, wherein the test control unit comprises: aplurality of quenching time measuring units configured to measure thestep duration of the step pulse; and a pulse generating unit configuredto generate the first step enable pulse and the full step enable pulsebased on output signals of the plurality of the quenching time measuringunits and output the first step enable pulse and the full step enablepulse during the activation period of the test enable signal.
 7. Thenonvolatile semiconductor device according to claim 6, wherein each ofthe plurality of quenching time measuring units comprises: a pulsedriving unit configured to drive the step pulse in response to an enablesignal; and a latch unit configured to latch an output signal of thepulse driving unit.
 8. The nonvolatile semiconductor device according toclaim 2, wherein the current measuring unit further comprises a delayunit configured to delay an initial enable signal to control an enableperiod of the current measuring unit.
 9. The nonvolatile semiconductordevice according to claim 1, wherein the current generating unitcomprises: an oscillator configured to generate a clock for determininga quenching duration of the set write current based on the referencecurrent; a clock counter configured to count the clock and output acount signal; a step pulse generating unit configured to generate thestep pulse in response to the count signal; and a write currentgenerating unit configured to generate the set write current dependingon the step pulse.
 10. The nonvolatile semiconductor device according toclaim 1, wherein the set write current maintains a given current valuefor a given time at an initial state and then changes to a current valuethat is decreased in a step form in response to the step pulse.
 11. Thenonvolatile semiconductor device according to claim 10, wherein a firststep period of the set write current has a given duration.
 12. Thenonvolatile semiconductor device according to claim 1, wherein thecurrent measuring unit measures the step duration using a first steppulse corresponding to a period from an initial state until a currentvalue of the set write current starts to decrease in a step form, asecond step pulse corresponding to a period ending when the set writecurrent decreases by one step, and a third step pulse corresponding to aperiod from the initial state until the current value of the set writecurrent decreases by a whole step.
 13. A method for testing anonvolatile semiconductor device, the method comprising: generating aset write current depending on a step pulse that is generated based on areference current and outputting the set write current to a memory cell;and measuring a step duration of the step pulse and outputting the stepduration outside of a chip during an activation period of a test enablesignal.
 14. The method according to claim 13, further comprisingmeasuring a quenching slope change of the set write current at theoutside of the chip based on the step duration.
 15. The method accordingto claim 14, further comprising: detecting a high level or a low levelof the step duration by applying a time duration to the step durationoutputted through an output pad; and checking a toggling point tomeasure the quenching slope change.
 16. The method according to claim14, wherein the measuring of the step duration comprises converting thequenching slope change of the set write current into a pulse.
 17. Themethod according to claim 16, wherein the measuring of the step durationfurther comprises outputting a first step enable pulse representing afirst step pulse duration of the step pulse and a full step enable pulserepresenting a pulse duration of whole steps of the step pulse.
 18. Themethod according to claim 13, wherein the generating of the set writecurrent comprises: generating a clock for determining a quenchingduration of the set write current based on the reference current;counting the clock to output a count signal; generating the step pulsein response to the count signal; and generating the set write currentdepending on the step pulse.
 19. The method according to claim 13,wherein the set write current maintains a given current value for agiven time at an initial state and then changes to a current value thatis decreased in a step form in response to the step pulse.
 20. Themethod according to claim 19, wherein a first step period of the setwrite current has a given duration.